1. Field of the Invention
The present invention generally relates to an evaporator system. More particularly, the present invention relates to a closed evaporator system for preparing samples for chemical analysis.
2. Description of the Related Art
In many industries, including the semiconductor fabrication industry, knowledge of the level of contaminates in a chemical solution and of the air in is the processing areas is extremely important. Analytical laboratories receive sample vials, or the like, containing samples of chemical solutions and of airborne contaminates dissolved in water or other solutions. In order to assay the samples for the type and quantity of contaminates, the samples are evaporated such that the relative concentration of the contaminates is increased ideally resulting in only solid matter remaining for the sample. It is usual for an analytical laboratory to ascertain that the laboratory's handling of a sample has not added further contaminates and the analytical procedures are accurate. To this end, the laboratory will include a known blank sample (no contaminates or a known type and amount of contaminates) and process the blank sample along with the actual samples. When the analysis is completed for the b lank sample, the laboratory can ascertain the type and quantity of any contaminates added to the sample due to processing within the laboratory. The laboratory will also process a control sample having a known quantity and type of solids to ascertain the accuracy of the analytical procedure. It is desirable to expose the blank and the control samples to the same conditions as possible as experienced by the samples to be analyzed.
The sensitivity of an analytical procedure is increased as the level of laboratory's contamination of the sample is decreased. For example, if the laboratory added contaminate X in the quantity of 500 ppm (parts per million) plus or minus 50 ppm, an analytical test for contaminate X can have no greater sensitivity than 50 ppm.
A present open evaporation system for evaporating samples in shown in FIG. 1. A hot plate 11 having heat control 12 is used for heating the sample to cause evaporation of the sample. The sample is placed in a high purity fluoropolymer, such as TEFLON.TM., bowl 14 which sits in a petri dish 13 which in turn sits upon hot plate 11. A high purity fluoropolymer gas input cover 15, commonly referred to as a thier, covers bowl 14 and receives a purified inert gas such as Nitrogen from an unregulated gas source through inlet 16. Overhead heat lamp 17 is provided to avoid condensation from occurring onto the thier 15. The entire system is located under a venting hood 18. The evaporation of the sample takes place in a purified inert gas, such as Nitrogen, atmosphere. The vapors from the evaporation process, as indicated by the arrows, flow from the sample into the chamber formed by the thier 15 and petri dish 13, out of that chamber from around the edge of the thier 15 contacting petri dish 13 and then up into the venting hood 18. Such an open evaporation system has an average evaporation rate of 15 ml/hr and a sensitivity of approximately 10 ppm. The actual evaporation rate is determined by the density of the chemical being evaporated.
An open evaporator system for processing a plurality of samples is discussed in U.S. Pat. No. 5,514,336 entitled "Automated Evaporator For Chemical Analysis". Here the samples are in open vials which are partly submerged in heated water to cause evaporation of the samples. A purified inert gas, such as Nitrogen, source is provided through a regulator to a manifold which distributes purified inert gas, such as Nitrogen, to each of the samples. The Purified inert gas, such as Nitrogen, helps to remove the solvent vapors and limits oxidizing of the samples by displacing oxygen from the air.
A closed negative pressure evaporator system for processing a single sample is discussed in U.S. Pat. No. 5,612,847 entitled "Dynamic Vacuum Evaporation System". Here the sample is in a closed vessel which is covered partially by a cover on five sides and exposed to a heater on the six side to cause evaporation of the samples. A purified inert gas, Argon, source is provided under negative pressure to the sample in the vessel while the sample is being evaporated. The purified inert gas, picks up the volatiles from the sample which is evacuated from the vessel by the vacuum pump. There is, however, some risk of explosion if the system is not operated properly.